Mercury as a trace element in coal becomes a contaminant in flue gas from coal-fired power plants and other coal fired furnaces. As a result, processes have been developed to capture mercury (Hg) contained in flue gas.
For example, one process that has been developed injects activated carbon into flue gas to absorb mercury. This process reports capture rates of up to about 90% of the total mercury contained in the coal. Unfortunately, activated carbon is expensive and thus its use for mercury removal adds significantly to overall costs.
In addition to the concerns of mercury in flue gas from coal-fired power plants, the presence of mercury on mixtures of fly ash particles and activated carbon slated for disposal has also been a significant regulatory concern because of the potential for ground water and surface water contamination.
Fly ash without activated carbon may be used as a partial replacement for Portland cement in concrete if it meets certain specifications (such as those found in ASTM C618-05 “Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete”). The most common reason fly ash without activated carbon cannot be used in concrete is excess unburned carbon content in the ash. Excess unburned carbon is not allowed because it absorbs additives used in concrete making and makes them ineffective. However, after addition of activated carbon for mercury capture, ash is generally unusable even if it meets the unburned carbon specifications. This is because the activated carbon absorbs the concrete additives to a much large degree than the unburned carbon normally found in fly ash.
Fly ash with excess unburned carbon or added activated carbon may be beneficiated to allow use in concrete by Carbon Burnout or other thermal fly ash beneficiation processes to substantially reduce the carbon content. Carbon Burn-Out technology is disclosed in U.S. Pat. No. 5,160,539 and U.S. Pat. No. 5,399,194, which are both herein incorporated by reference in their entirety. Thermal beneficiation processes such as Carbon Burn-Out are known to preferentially combust the most adsorptive carbon particles (typically those with high surface areas such as activated carbon injected for mercury capture).
As disclosed in “Treatment of Mercury in Fly Ash by the CBO™ Process,” by Joe Cochran and Vincent Giampa, Research Disclosure Journal, June of 2003, which is herein incorporated by reference in its entirety, carbon burn-out technology can be configured for two different mercury treatment configurations.
With respect to the first configuration, the high carbon fly ash particles are conveyed from a silo to a fluid bed combustor where a fan provides fluidization and combustion air to the fluid bed combustor. “Fly ash particles” as used herein means the combination of the mineral portion of the fly ash plus unburned carbon which may be attached to or separate from the mineral portion as well as any activated carbon or other particulate additive which has become part of the fly ash. In the fluid bed combustor, carbon in the fly ash particles combusts on a continuous basis and the product fly ash particles and flue gas exit the fluid bed combustor with the mercury vaporized in the flue gas so that the beneficiated fly ash particles are essentially mercury free. The beneficiated fly ash particles and flue gas are cooled by heat exchange with condensate of the power plant (or other heat exchange medium) to temperatures between 300° F. and 550° F. During cooling, the volatized mercury which initially was with the fly ash particles is no longer vaporized and returns back to the beneficiated fly ash particles. These cooled beneficiated fly ash particles are separated from the flue gas, which is now essentially mercury free, by a cyclone and a baghouse (or particulate separation devices of similar function). The separated, beneficiated fly ash particles with the mercury are conveyed to a storage and load-out area for use as a direct replacement for Portland cement. After use of the beneficiated fly ash particles in concrete, the mercury is substantially sequestered within the concrete matrix.
With respect to the second configuration, the exhaust with the vaporized mercury which leaves the fluid bed combustor is processed. Two options exist for processing this exhaust to remove the mercury.
With the first option, the exhaust is cooled to a suitable temperature well below the condensation temperature of mercury, such as between 300° F. and 550° F., to a temperature suitable for passing through a conventional baghouse. The mercury is condensed or absorbed on the small mass of unbeneficiated fly ash particles. These unbeneficiated fly ash particles elutriated from the fluid bed combustor have a high carbon content of 25% to 50% which is believed in the prior art to aid in mercury capture, similar to the use of activated carbon. Next, these unbeneficiated fly ash particles containing mercury are forwarded to a mercury recovery process and the cleaned exhaust gas can be used for other operations.
With the second option, the exhaust is cooled to the lowest temperature at which substantially all the mercury remains in the gas phase which is about 1100° F. and then the elutriated fly ash particles are separated in a high temperature baghouse or other particulate capture device of similar purpose. The separated fly ash particles are returned to the fluid bed for further carbon reduction while the flue gas containing the vaporized mercury is forwarded to an on-site mercury recovery process. After removal of mercury, with for example activated carbon, the flue gas may be returned for other operations.
The first configuration described above discloses that mercury which was initially in the fly ash particles is not lost and ultimately remains with the beneficiated fly ash particles after the carbon burn-out process. The second configuration describes a first option where vaporized mercury is condensed or absorbed on high carbon content, unbeneficiated fly ash particles and a second option where the vaporized mercury is separated from the fly ash particles and is forwarded to a mercury recovery process.
Accordingly, it is known that mercury may be captured from the boiler exhaust gas by injection of activated carbon. Additionally, it is known that mercury initially in unbeneficiated fly ash particles which are subjected to carbon burnout or other thermal fly ash beneficiation processes will ultimately remain with the beneficiated particles. Further it is known that during the thermal fly ash beneficiation processes, the vaporized mercury can be condensed or absorbed on unbeneficiated fly ash particles having a high carbon content of 25% to 50% or the flue gas with the vaporized mercury can be separated from the fly ash particles for a subsequent mercury removal process.
As a result, even with thermal fly ash beneficiation processes, the prior art teaches that to remove mercury from exhaust, if material is added to exhaust gases to capture mercury that material needs to have high carbon content. Unfortunately, the use of activated carbon to capture mercury is expensive and its addition in fly ash requires additional thermal beneficiation to makes the resulting fly ash usable. Accordingly, there is clearly a need not taught or suggested by the known prior art discussed herein for an effective and low cost alternative for capturing new mercury emissions in exhaust streams from coal fired burners and boilers.